Cemented carbide, also called hard-metal, is class of hard material comprising a hard phase of metal carbides and/or carbo-nitrides, the metal being selected from groups IVa to VIa of the periodic table and a metallic alloy binder comprising one or more iron-group metals. Hard-metals are produced by a powder metallurgy method typically including the steps of milling, mixing, pressing and liquid-phase sintering. The sintering temperatures of the most commonly used WC—Co hard-metals are usually above the melting point of a eutectic temperature, which is in the range of about 1300 deg. C. to 1320 deg. C. The sintering temperatures used for another class of hard-metals called cermets and comprising TiC or TiCN with a Ni—Mo-based binder, are above the melting point in the Ti—C—Ni—Mo system of roughly 1280 deg. C.
Typically the sintering temperatures for hard-metals are above 1350 deg. C., which allows the formation of a large fraction of liquid phase during sintering in order to promote full density of the sintered product.
The term “wear part” is understood to mean a part or component that is subjected, or intended to be subjected to wearing stress in application. There are various kinds of wearing stress to which wear parts may typically be subjected such as abrasion, erosion, corrosion and other forms of chemical wear. Wear parts may comprise any of a wide variety of materials, depending on the nature and intensity of wear that the wear part is expected to endure and constraints of cost, size and mass. For example, cemented tungsten carbide is highly resistant to abrasion but due to its high density and cost is typically used only as the primary constituent of relatively small parts, such as drill bit inserts, chisels, cutting tips and the like. Larger wear parts may be used in excavation, drill bit bodies, hoppers and carriers of abrasive materials and are typically made of hard steels which are much more economical than cemented carbides in certain applications.
In order to prolong the working life of steel wear parts it is common for the wear parts to have hard facings, which are coatings of a harder material attached to the surface of a body, in this case, the wear part. Hard facings may be applied repeatedly to a wear part as previous hard facings wear away, thereby repeatedly restoring the wear part to a usable condition. There are various hard facing materials and methods known in the art. Welding, brazing and spraying of hard particles are examples of widely used methods.
In the welding method, a weld strip or rod comprising a welding alloy and grains of hard or super-hard materials is prepared and subjected to localised heating proximate a wear part surface, causing a portion of the wear part surface to melt and become metallurgically bonded to the hard facing. Hard facing methods which involve the formation of metallurgical bonding with the wear part (substrate) surface require heat to be applied to the wear part surface in order to raise its temperature to a level at which the bond can form. For example, in welding methods the heat may be applied by means of an electrical arc or current. The applied heat may result in the degradation or melting of a steel substrate. The minimum temperature that can be used depends on the composition of the hard facing. Where meta-stable, ultra-hard materials such as diamond grains are incorporated into a hard facing as is known in the art (see, for example, U.S. Pat. Nos. 5,755,299, 5,957,365, 6,138,779 and 6,469,278), the applied heat may substantially degrade important properties of those ultra-hard materials.
In the spraying method, a powder comprising a hard phase, typically tungsten carbide, is caused to impact the wear part surface with high energy, resulting in a dense layer of mechanically keyed hard particles becoming attached to the surface. Sprayed coatings typically do not from metallurgical bonds with the substrate surface unless the coatings have been treated at high temperature, which is typically necessary in order to increase the coating density and reduce or eliminate porosity. If the coatings comprise WC—Co, it may be necessary to treat the coating at high temperatures exceeding about 1,350 deg. C. Such high temperatures may result in the distortion or melting of the steel substrate body, which is highly undesirable. Another disadvantage of thermal spraying methods, such as flame, plasma or high velocity oxy-fuel (HVOF) spraying, is that they require expensive specialised equipment.
The direct sintering of hard-metal powders onto steel substrates has the potential of being relatively simple and economical. Unfortunately, this method is not practicable owing to the fact that the hard-metal shrinks during the sintering process, resulting in an inhomogeneous structure and severe cracking of the sintered layer (hard facing). Another major problem is the need to apply high temperature to the layer and steel substrate.
US Patent Publication No. 2007/0092727 teaches a wear part comprising diamond grains, a carbide phase such as tungsten carbide and a metallic alloy with liquidus temperature less than 1,400 deg. C. and preferably less than 1,200 deg. C. Two methods are taught for making the wear parts. In the first method an intermediate article comprising diamond grains is contacted with a source of both a selected infiltrant first alloy and a selected second alloy, the temperature of the source and intermediate article is raised to above the liquidus of the infiltrant alloy, causing the latter to infiltrate into the pores of the intermediate article. The time required for the temperature to be maintained above the liquidus is said to be about 15 minutes. Carbides are formed when components of the second alloy react with the diamond of the intermediate article. In the second method, which is more suitable for making larger wear parts, an intermediate material comprising diamond grains and an alloy selected from the first group and an alloy from the second group is subjected to hot pressing at a temperature lower than 1,200 deg. C. No infiltration is required in the second method.
This US patent publication also teaches a method for making diamond-containing wear parts using an alloy with a relatively low melting point, resulting in relatively less diamond degradation during manufacture. The economical viability of wear parts made according to these teachings is constrained by the cost of having a high content of diamond and other costly materials such as tungsten and other refractory metals throughout the body of the part, whereas such materials are typically necessary at the wear surfaces only.
Stainless steel alloys developed for the nuclear industry are taught in U.S. Pat. No. 5,660,939 and UK Patent No. 2,167,088, for example, and comprise chromium, nickel, silicon and carbon, but positively do not contain cobalt, which is generally unsuitable for use in a radio-active environment. These alloys are both wear and corrosion resistant.
U.S. Pat. No. 3,725,016 describes a method of coating a steel substrate with a hard metal coating. The coating is produced by spraying the components for the coating on to a surface of a steel substrate drying the coating and then raising the temperature of the coated steel substrate to a temperature above the liquidus temperature of the binder components of the coating. This elevated temperature is maintained for about half an hour. This long sintering time will result in considerable melting of both the binder components and the steel substrate.
There is a need to provide economically viable wear parts, more especially large wear parts comprising steel which parts exhibit enhanced wear behaviour. In particular, there is a need to coat or clad steel wear parts with a material that is more wear resistant than steel and which material is well bonded to the steel part, in order to prolong the working life of the part, rather than replace the steel part with one made substantially or entirely from a more expensive material. This is particularly so for steel wear parts which have non-planar or complex surfaces.